As a positive-working photoresist composition (hereinafter referred to as a "photoresist"), a composition containing an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive material is generally used. For example, novolak type phenol resin/naphthoquinonediazide-substituted compounds are described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470; and cresol-formaldehyde novolak resin/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfonic acid esters are described in L. F. Thompson, Introduction to Microlithography, No. 219, pp. 112-121 (American Chemical Society, 1983).
Novolak resin is useful as a binder for a photoresist because it dissolves without swelling in an aqueous alkali solution and when used to form a mask for etching it provides high durability or resistance to plasma etching.
The 1,2-naphthoquinonediazide compounds which are used as photosensitive material, function as dissolution inhibitors for reducing the alkali solubility of a novolak resin. But when these compounds are decomposed by irradiation with light they form alkali-soluble material which acts to sharply increase the alkali solubility of a novolak resin. This change in solubility resulting from exposure to light makes these naphthoquinonediazide compounds particularly useful as photosensitive materials for a positive-working photoresist.
Many positive-working photoresist compositions containing a novolak resin and a naphthoquinonediazide series photosensitive material have been developed and practically used.
Although such photoresists are characterized by having the high resolution described above, they tend to be affected by light reflected by a substrate with high reflectivity, such as aluminum (or the influence of halation). This results in the formation of a blurred image or causes difficulty in controlling the line width of the image. These problems are exacerbated when the substrate has fluctuations in the surface level.
In order to prevent halation, the addition of a light-absorbing material has been proposed. For example, JP-B-51-37562 (the term "JP-B" as used herein refers to an "examined Japanese patent publication") discloses a method for preventing the decrease in resolution which comprises reducing the permeability of light into a photoresist layer by incorporating Oil Yellow of the formula: ##STR7## Oil Yellow is a dye having light absorption characteristics in the ultraviolet region. As a light absorber, Oil Yellow decreases the light reflected back from the substrate surface into the photoresist layer. Thus, light that would be refracted into areas which should not be irradiated with ultraviolet rays is reduced and the resulting reduction in resolution is prevented.
However, the use of Oil Yellow still has defects. A part of the dye used as a light absorber is sublimed from the photoresist film during the prebaking conducted to remove the residual solvent and heightens its adhesiveness to the substrate after coating. This sublimation results in the halation-preventing effect being significantly lowered.
JP-A-55-36838 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") discloses 1-alkoxy-4-(4'-N,N-dialkylaminophenylazo)benzene derivatives as light absorbers that have improved sublimation resistance during prebaking. However, incorporation of these light absorbers into conventional positive-working photoresists is not advantageous since it causes a significant reduction in the sensitivity of the resulting photoresists.
The alkali-soluble azo compounds disclosed in JP-A-59-142538 do not have the disadvantage of causing a reduction or wide fluctuation in sensitivity, but these compounds are still insufficient to prevent halation. In addition, they cannot resolve the rapidly diminishing dimensions required by the semiconductor industry.